My talk with each was largely “Treat your partner with respect. Make sure you use birth control.”
ShawWife helped ShawD with birth control.
When they had SOs, we let them know that we were comfortable with them sharing a bedroom in our home (especially since they were sharing a bed at the dorm or apartment). One of ShawSon’s GFs was scared to do so because that would have been third rail stuff in her family.
Our feeling was like @Lizardly: Assume this one might be the one.
With ShawSon’s last GF, we didn’t think it would be a great choice for him. So, when I was visiting, I said, “If you want to marry her, we will be 100% behind you. But, if you are not sure, I see several red flags.” I went through them. He wasn’t totally happy with me, but they split up at the beginning of the pandemic. He started dating someone else and is now engaged. She is pretty fantastic – a big improvement in all dimensions.